This invention relates to the protection of power transformers from excessive heating and, in particular, to controlling the flow of a coolant (e.g., oil) in and about the primary and secondary windings of power transformers.
Power transformers of interest are of the type which, for example, are used in substations and are designed to be part of a high voltage electric power transmission and/or distribution system. These transformers are very expensive and can be easily damaged by excessive heat. It is therefore highly desirable that they be operated so they do not overheat. To this end, large power transformers are generally located within tanks filled with a suitable cooling liquid (e.g., oil) and pumps (e.g., oil pumps) are used to circulate the cooling fluid contained within the transformer's tank.
Certain large power transformers, which may, or may not, be contained within liquid filled tanks need to be cooled as soon as the transformer is energized. These transformers have “no self-cooled” rating. They can not be safely operated without the application of some coolant. Hence, pumps causing a coolant to circulate about these transformers must be started as soon as the transformer is energized. These power transformers with a no self-cooled rating should not be energized unless cooling pumps can be reliably started upon energization of the transformer. With transformers of this design, the failure to start the pumps upon energization can result in overheating and irreversible damage to the transformer.
Note that when a transformer is first energized there may be a momentary inrush of current and various circulating currents in and about the transformer which cause a sudden rise in the temperature of the transformer. Therefore, it is imperative that the cooling fluid be made to circulate simultaneously with the energizing of the transformer to handle heat and temperature conditions due to energization of the transformer.
Prior art methods to handle the problem of turning on the motors driving the cooling pumps and the cooling fans at the right time to circulate the coolant about the transformers include sensing the transformer voltage through: (1) built-in bushing potential devices and/or (2) using a breaker status contact within a circuit breaker used to apply power to the transformer. There are several drawbacks with these prior art methods as noted below.
Known built-in bushing potential devices can detect the application of an operating voltage to a power transformer and in response thereto control a relay to turn-on the pump motors. For example, a prior art method used to activate (turn-on) cooling pumps when power is applied to the transformer (i.e., “energizing” the transformer) includes connecting the output of a Bushing Potential Device (e.g., a KA-108 device sold by General Electric) to a cooling pump motor contactor which provides power to, and turns on, the cooling pump motor. Bushing Potential Devices produce approximately 120 Volts at 60 or 50 Hz when voltage is applied to the transformer. The problem with using Bushing Potential Devices is that they are prone to failure. When they fail, they de-energize the cooling pumps or do not provide the necessary potential to the cooling motors to drive the cooling pumps when power is applied to the transformer. As a result the extremely expensive transformer can be subjected to excessive heat and suffer significant damage.
The breaker status contact (e.g., contact 52A or 52B) indicates that the circuit breaker used to energize the transformer is closed or tripped, whether or not the circuit breaker itself is energized. Using circuit breaker status (e.g., contact 52A) is generally reliable. But such use can cause the pumps to start circulating the cooling fluid before the transformer is energized. Should this happen, starting the pumps prematurely could over-cool the liquid cooling medium. This in turn can lead to degradation of, or damage to, the pump. Over-cooling the liquid cooling medium can also lead to static electrification which in-turn may lead to catastrophic transformer failure. Another problem with this method is that the owners and/or operators of the transformer may test the circuit breakers when the line is not energized. This requires that the operator disable the pump control circuit until testing is complete. If the pump motor and the associated pump are not disabled during testing, the cooling pump will continue to run. This may result in over cooling the insulating fluid, which could lead to mechanical failure of the pump. Thus, known methods of cooling power transformers, particularly the ones with a no-self cooling rating, though generally effective do not always function as reliably as desired.
The problem of supplying cooling by the timely and reliable activation of fans and pumps is not limited to those with a no-self cooling rating. It also applies to many other types of power transformers, such as those whose power handling rating is a function of their temperature under certain power conditions.
Therefore, a need exists to reliably start the motors driving pumps and fans to cause a coolant to flow about a transformer as soon as the transformer is energized and to de-energize the motors and disable the pumps and fans when the transformer is de-energized. In essence, the problem is to turn-on the cooling system for a transformer in a timely and reliable fashion and to turn off the cooling system in a timely and reliable fashion.